November / December Media Highlights: The Geological Society of America Bulletin

Boulder, CO, USA - Geology topics of interest include new insights into evolution of Baja California and Mount Diablo, a large, actively growing fold in Earth’s crust between two major faults in the San Andreas system in the San Francisco Bay area.

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The structure and rate of late Miocene expansion of C4 plants: Evidence from lateral variation in stable isotopes in paleosols of the Siwalik Group, northern Pakistan

Behrensmeyer et al.’s research pioneers a new approach to the study of change through time in ancient landscapes using lateral variation in stable carbon and oxygen isotopes in minerals precipitated in ancient soils. They examined successive alluvial landscapes during a period of marked climate change in the sub-Himalayan region of northern Pakistan. Measures of carbon and oxygen stable isotopes in calcium carbonate precipitated in buried floodplain soils reveal changes in the vegetation that grew upon these soils. Many successive soils were formed and buried during the accumulation of thousands of meters of sedimentary debris eroded from the rising Himalayan mountains. Their results document an overall trend from forest to grassland between 8.0 and 5.0 million years ago, interpreted as a response to intensified monsoons and reduced total rainfall. Lateral variation in isotopes within 11 of the ancient soils shows that this 3 million year transition interval was characterized by a mosaic of forested and grassy habitats that conformed to drier and moister areas on the alluvial plain. Local topography and substrate therefore helped to moderate the effects of overall climate change in this region of southeastern Asia for a long period of time, allowing the fauna and flora to adapt to increasing seasonal aridity and open grassland vegetation.
Keywords: carbon isotopes, oxygen isotopes, paleosol, Miocene, Pakistan.
***************Pyrite-enhanced chemical weathering in Kärkevagge, Swedish Lapland

Conventional wisdom once held that weathering in cold climates was overwhelmingly due to physical processes. In 1960, this view was challenged by Anders Rapp in his publication on landscape denudation in Kärkevagge, Swedish Lapland, where he made the startling discovery that chemical weathering exceeded any measured physical denudation process. His interpretation was based on limited analyses of water chemistry where he found total flux of dissolved solids accounted for most mass loss from the watershed. The dominant anion he observed was sulfate. He also observed other features of chemical weathering in the valley, including white streaks of lime in stream channels on the valley flanks. While insightful, Rapp offered no mechanism to explain these findings. Darmody et al.’s subsequent work in Kärkevagge has revealed the driving mechanism of chemical weathering to be acid production from pyrite oxidation. They have determined that pyrite occurs in some of the rock units in the valley and its oxidation produces sulfuric acid, H2SO4, which accelerates weathering. The “lime coats” are primarily an amorphous aluminum oxyhydroxide which paints surfaces it contacts. That this is an active feature is demonstrated by efflorescence on actively growing seasonal vegetation in stream channels. In sheltered overhangs, protected from wind and running water, Darmody et al. found other secondary minerals associated with pyrite oxidation, including white crusts of gypsum, yellowish coatings of jarosite, KFe3(SO4)2(OH)6, and rust-colored ferrihydrite, Fe+32O3·0.5(H2O). They believe that pyrite oxidation may be an important first step in understanding weathering in many environments. It largely goes unrecognized because it occurs rapidly and typically is only identified in highly disturbed landscapes associated with mining and other large-scale earth-moving activities. An additional implication of their findings is that Rapp may have chosen a poor location (where the particular geochemistry evokes accelerated chemical weathering) to do his work.
Keywords: weathering, pyrite, Sweden, Arctic, efflorescence, rock coating, scanning electron microscopy.

In Drenth et al.’s paper, the three dimensional geometry of the Pine Canyon caldera in the Chisos Mountains of Big Bend National Park is studied using high-resolution aeromagnetic data. The caldera has long been interpreted as a classic example of a downsag caldera, with no major collapse faults exposed on the surface. Geophysical evidence for buried faults is detected, as well as evidence of asymmetric collapse, making the Pine Canyon caldera one of the first downsag calderas to be described in terms of its geophysical expression. A broad magnetic anomaly observed over the entire Chisos Mountains is interpreted to be sourced by a large intrusion: 28–32 km in diameter, 1–4 km thick, and 700–3000 km3 in volume. This makes it by far the largest intrusion in the region and probably the easternmost major expression of extensive Tertiary magmatism in the southwestern United States. Northeast-southwest elongation of the intrusion is parallel to major trends of the Paleozoic Ouachita orogeny, indicating possible inherited structural control on intrusion emplacement location and style.
Keywords: magnetic anomalies, Trans-Pecos Texas, Pine Canyon caldera, intrusions, magnetic exploration, ash-flow tuff.

*************Ridge-trench interactions and the Neogene tectonic evolution of the Magdalena shelf and southern Gulf of California: Insights from detrital zircon U-Pb ages from the Magdalena fan and adjacent areas

Approximately 15 million years ago, a series of oceanic spreading centers over 1000 km long had approached and become adjacent to the long-lived subduction zone that used to exist along the western margin North America. This placed the hottest known tectonic environment adjacent to the coldest known tectonic environment and caused a major reorganization of the plate margin, including the eventual severing of the Baja California Peninsula from the mainland. It is known that Baja California formed a small tectonic plate with major fault zones on either side. Fletcher et al.’s study, however, shows that previous assumptions for how much shearing had taken place in each fault zone are fundamentally wrong. The main objective of this study is to identify the source terrane for a large accumulation of sedimentary debris on the seafloor west of Baja California and thus measure the displacement across intervening faults. The Fletcher et al.’s new kinematic model for shearing across the margins of the Baja California microplate requires a major rethinking of the processes that led to the tectonic plate reorganization that started approximately 15 million years ago west of Baja California.
Keywords: Magdalena fan, detrital zircon, U-Pb, Baja California, Integrated Ocean Drilling Program, Gulf of California, Neogene.

The Waipaoa sedimentary system, North Island, New Zealand, which spans ~100 km from terrestrial upland to continental rise, is a spectacular natural laboratory, where rates of geomorphic activity are amongst the highest recorded globally. Gomez et al. used sediment cores to examine the relationship between terrestrial erosion and marine deposition during the last 2400 years. Three inter-correlated terrestrial and marine sedimentary records preserve events that strongly reflect changes in the adjacent terrestrial environment and are related to human activity, climate, and vegetation changes and extreme events, such as storms and subduction-thrust earthquakes. Extreme storms are the minimum geomorphologically effective event preserved in the sediment records, but deforestation of the hinterland in the nineteenth century impacted sediment production and dispersal across the entire magnitude–frequency spectrum of events regulating sediment delivery to and transport in stream channels.
Keywords: sediment dispersal, sediment cores, source to sink, depositional signals, environmental change.

Gross et al.’s study provides new insight into the development, classification and distribution of throughgoing fractures in sedimentary rocks. These large fractures span multiple stratigraphic beds and may serve as significant pathways for the flow of fluids in the subsurface. Fracture data collected from limestone outcrops in southern Israel demonstrate that throughgoing fractures in thin-bedded carbonate rocks develop by the linkage and coalescence of smaller fractures. Further, the population of throughgoing fractures represents a continuum of structures at various stages of development, which can be classified into geometric categories based on increasing levels of deformation. Such differences can be used to map variations in fracture intensity across large geologic structures such as folds and fault zones. Results from this study contribute to the characterization of fractured reservoirs and aquifers, and thus have direct application to oil and gas exploration and water resource evaluation.
Keywords: fractures, fracture zones, brittle deformation, joints, connectivity, Israel.

***************Stateline fault system: A new component of the Miocene-Quaternary Eastern California shear zone

The 200-km (125 miles)-long Stateline fault system is a right-lateral strike-slip fault zone with clear Late Quaternary surface ruptures extending along the California-Nevada state line, from Primm, Nevada area along Interstate 15 to the Amargosa Valley. The fault passes within 40 km of the Las Vegas strip, 10 km of the town center of Pahrump, Nevada, and appears to end near the town of Amargosa Valley, Nevada (about 40 km west-southwest of the site of the proposed high-level nuclear waste repository at Yucca Mountain). This fault has long been considered inactive and of only minor importance to the tectonic pattern of eastern California and southwestern Nevada, whereas fault systems like the Death Valley, Panamint Valley, and Owens Valley have received much more attention. New research focused on the Stateline fault system is beginning to change how we view this fault zone. Guest et al. present geologic data that establishes the minimum offset on the southern segment of the fault system to be 30 ± 4 km over the last 13 million years. This implies a minimum average slip rate for the southern segment of the fault system of 2.3 ± 0.35 mm/yr. This is twice the slip rate estimated from geodetic monitoring in the region, and therefore the fault is either in a transient period of slow slip or has been abandoned as activity in the eastern California shear zone has migrated west. The magnitude of accumulated offset, evidence for Late Quaternary slip, and rapid long-term slip rate indicate that the Stateline fault system is a major component of the Eastern California shear zone. Given its proximity to population centers and important infrastructure in southern Nevada, the fault warrants close scrutiny in seismic hazards analyses of the region.
Keywords: Eastern California shear zone, stateline fault, tectonics, fault offset, fault slip rate, slip rate discrepancy.

Western Amazonian geological history has been a subject of intense debate during the last ten years, and nearly opposite theories (continental versus marine) have been proposed regarding the depositional nature of Miocene Amazonia. To help address this controversy, Hovikoski et al.’s paper presents a large sedimentological and ichnological data set that spans the Lower-Upper Miocene strata in the area. The data show that several tens of high-frequency marine ingressions reached the basin during the Miocene. The marine incursions were shallow and restricted, and were interspersed with rapidly advancing coast lines. Along with the prograding shorelines, the continental environments—swamps, lagoons, floodplains and forests—constrained the extent of the marginal marine embayment. The repetitive nature of the ingressions provides a logical explanation for the species composition of the modern Amazonian rivers.
Keywords: sedimentology, ichnology, tidal deposits, brackish water, western Amazonia, Miocene.

***************40Ar/39Ar and field studies of Quaternary basalts in Grand Canyon and model for carving Grand Canyon: Quantifying the interaction of river incision and normal faulting across the western edge of the Colorado Plateau

40Ar/39Ar dates on basalts of Grand Canyon provide one of the best records in the world of the interplay among volcanism, differential canyon incision, and neotectonic faulting. Earlier 40K/40Ar dates indicated that Grand Canyon had been carved to essentially its present depth before 1.2 Ma. But new 40Ar/39Ar data cut this time frame approximately in half; new ages are all <723 ka, with age probability peaks at 606, 534, 348, 192, and 102 ka. Strategic sampling of basalts provides a semicontinuous record for deciphering late Quaternary incision and fault-slip rates and indicates that basalts flowed into and preserved a record of a progressively deepening bedrock canyon.
Differential incision data provide evidence for relative vertical displacement across Neogene faults of the Colorado Plateau-Basin and Range transition, a key data set for evaluating uplift and incision models. Our data indicate that the Lower Colorado River block has lowered 25–50 m/Ma (150–300 m) relative to the western Grand Canyon block and 125–150 m/Ma (750–900 m) relative to the eastern Grand Canyon block in 6 Ma. The best model explaining the constrained reconstruction of the 5- to 6-Ma Colorado River paleoprofile, and other geologic data, is that most of the 750–900 m of relative vertical block motion that accompanied canyon incision was due to Neogene surface uplift of the Colorado Plateau.
Keywords: Grand Canyon, river incision, Ar-Ar dating, quaternary basalts, tectonic geomorphology.

Lewis et al.’s paper reports on the style and timing of one of the most significant events of late Cenozoic global-climate change, namely, the thermal transition from wet-based to cold-based alpine glaciation in Antarctica. Lewis et al. demonstrate that in the western Olympus Range, this transition was associated with a drop in mean annual atmospheric temperatures of ~25 °C, that this shift occurred prior to 13.94 Ma, and that the shift was followed directly by the most significant expansion of East Antarctic ice in the last circa 17 Ma. Chronologic control comes from 40Ar/39Ar analyses of in-situ ash fall deposits. The dating links, for the first time, onshore and offshore records detailing climate evolution during this crucial interval.
Keywords: alpine glaciers, geomorphology, Dry Valleys, Miocene, till, volcanic ash, Antarctica, Olympus Range, cold-based glaciers.

***************Millennial-scale dynamics of valley fills over the past 12,000 14C yr in northeastern New Mexico, USA

The moisture regime of the western United States is the result of annual-to decadal-scale climate fluctuations embedded in millennial-scale ones that have occurred since at least the end of the last ice age. Mann and Meltzer used radiocarbon-dated charcoal to describe the timing of cycles of valley incision and aggradation in northeastern New Mexico over the past 12,000 years. Their results show that nine periods of valley aggradation separated by incision episodes occurred during this interval. Changes in precipitation tied to the strength of the North American monsoon system probably drive these cycles of aggradation and incision. When the monsoon is strong, more frequent summer thunderstorms cause increased flooding in small watersheds. Wetter summers spanning from decades to centuries allow forest vegetation to spread, which reduces sediment input from hill slopes at the same time floods are eroding away valley fills. Aggradation of valley fills probably occurs when summer thunderstorms are less frequent and large floods are correspondingly rare. Prolonged summer drought reduces forest cover, which increases erosion from slopes and causes sediment to accumulate in valleys. The geomorphic cycles Mann and Meltzer describe reflect a previously unrecognized, millennial-scale climate oscillation important in shaping the landscapes of the southern Great Plains. Depending on the phase of this millennial oscillation, human-caused changes in the environment may be dampened or enhanced. The millennial cycle is currently in its moist, incision phase, but in the past such incision phases have been short-lived compared to the dry phases.
Keywords: alluvial stratigraphy, valley fills, Younger Dryas, Holocene, millennial scale, climate change, southwestern United States, High Plains.

***************The influence of large landslides on river incision in a transient landscape: Eastern margin of the Tibetan Plateau (Sichuan, China)

Ouimet et al.’s paper explores a probabilistic, numerical model to provide a quantitative framework for evaluating how landslides influence bedrock river incision and landscape evolution. The time-average number of landslide dams along a river course, and thus the magnitude of the landslide influence, is set by two fundamental timescales—the time it takes to erode landslide deposits and erase individual dams, and the recurrence interval of large landslides that lead to stable dams. Stable, gradually eroding landslide dams create mixed bedrock-alluvial channels with spatial and temporal variations in incision, ultimately slowing long-term rates of river incision, thereby reducing the total amount of incision occurring over a given length of river. A stronger landslide effect implies that a higher percentage of channel length is buried by landslide-related sediment, leading to reduced river incision efficiency. The longer it takes a river channel to incise into a landslide dam and remove all landslide-related sediment, the more control these events have on the evolution of the river profile and landscape evolution. This can be the result of slow erosion of stable dams, or a higher frequency of large events.
Keywords: Asia, Tibetan plateau, geomorphology, river incision, landslides, transient rivers, landscape evolution, Dadu River.

***************Flow lobes in granite: The determination of magma flow direction in the Trawenagh Bay Granite, northwestern Ireland, using anisotropy of magnetic susceptibility

This paper reports the discovery of “frozen” flow lobes in a ca. 400 Ma granite pluton from northwest Ireland. Tongue-like structures less than 1500 m wide, elongated east-west, were defined using magnetic fabric data obtained from anisotropy of magnetic susceptibility measurements (AMS). AMS was shown to represent the alignment of mineral grains that were aligned when the magma was not fully crystallized (in a “magmatic-state”). Therefore, these lobes were interpreted as flow structures—flow lobes. Because each lobe closes westward, they indicate that magma flowed into this pluton from east to west. This showed that the magma flowed out of an adjacent pluton situated within a major synplutonic shear zone. Stevenson et al. provide the first documented example of subunit scale flow lobes in a granite pluton, use AMS to determine the inflow direction of granitic magma, and demonstrate magma ascent through a shear zone, exiting the shear zone at the emplacement site.
Keywords: granite emplacement, anisotropy of magnetic susceptibility, AMS, flow lobes, shear zone, Trawenagh Bay Granite, Main Donegal Granite.

**************Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California

Mount Diablo is a large, actively growing fold in Earth’s crust that has formed between two major strike-slip faults of the San Andreas system in the eastern San Francisco Bay Area, California. Uplift of Mount Diablo during the past 3 to 5 million years has produced unique 3-D exposures of normal faults that were active in a marine forearc basin during late Cretaceous–early Tertiary time, coeval with plate convergence and subduction beneath western California. Stepwise restoration of Mount Diablo anticline and other late Cenozoic structures that thinned the basement rocks beneath the basin, and presently juxtapose rocks of the Franciscan complex that were metamorphosed at depths of 20 to 30 km (i.e., blueschist facies) with marine sediments that were never buried any deeper than about 4 to 8 km. Apatite fission-track analyses indicate that the Franciscan rocks were exhumed and cooled from depths of 20 to 30 km in the subduction zone while normal faulting and extension were occurring in the overlying forearc basin. The uniquely exposed structural relationships at Mount Diablo support models for exposure of Franciscan blueschists through syn-subduction extension and attenuation of the overlying forearc crust rather than uplift and erosion of the accretionary prism.
Keywords: blueschist, exhumation, subduction, attenuation, tectonics.

************** Rapid exhumation and cooling of the Liaonan metamorphic core complex: Inferences from 40Ar/39Ar thermochronology and implications for Late Mesozoic extension in the eastern North China Craton

The Liaonan metamorphic core complex formed during crustal extension in the Liaodong Peninsula. Its footwall mylonite zone is 2.5–3.5-km thick and includes Early Cretaceous (128-118 Ma) granitic rocks, together with older metamorphic rocks. Exhumation of its footwall from mid-crustal levels is evidenced in the detachment fault zone by the temporal transition from amphibolite-facies mylonitization at depth, through retrograde chloritic shearing and brecciation, to brittle faulting during final uplift. 40Ar/39Ar ages of muscovite, hornblende, biotite and K-feldspar from the mylonitic rocks record that the core complex cooled between ca. 120-107 Ma from the time of initial crystallization of zircons (122-118 Ma) at 700~800 °C in syn-tectonic leucocratic dikes and granitic rocks, to closure of argon diffusion in hornblende, micas, and K-feldspar at ~500 °C to ~200 °C. Throughout the eastern North China Craton, the synchroneity of cooling and exhumation of metamorphic core complexes, the formation of pull-apart basins, and regional alkaline igneous activity reflects regional extensional tectonics in the Early Cretaceous. The accompanied lithospheric thinning possibly resulted from the rollback of the subducted Pacific plate along the eastern Asian margin during the Early Cretaceous.
Keywords: argon thermochronology, metamorphic core complex, crustal extension, Early Cretaceous, North China Craton, Liaodong Peninsula, Liaonan metamorphic core complex.

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